The space multiplexing MIMO communication system transmits signals using multiple antennas of the receiving side and the transmitting side, and provides a feature of high throughput. FIG. 1 shows a configuration of the conventional MIMO communication system. As shown in FIG. 1, in the space multiplexing MIMO communication system, transmitter 10 transmits different symbol sequences s1, s2 . . . , sN through transmitting antennas 11-1 to 11-N, respectively, at the same time, in the same symbol period, and receiver 20 receives the signals using receiving antennas 21-1 to 21-M. Channel fading between transmitting antennas 11-1 to 11-N and receiving antennas 21-1 to 21-M is h11, h12, . . . , hMN, respectively. Receiver 20 detects signals r1(n), r2(n), . . . , rM(n) received at the same time, by utilizing the feature that channel fading from one transmitting antenna is not influenced by channel fading from the other.
For example, there is a technique of demultiplexing a plurality of signals transmitted at the same time through processing in chip units by utilizing one receiving antenna at the receiving side in an open-loop MIMO communication system (see Non-Patent Documents 1, 2, 3 and 4). The technique will be described below.
FIG. 2 shows a configuration of the transmitting side of a conventional MIMO communication system employing the MPD (Multi Path Diversity) method.
First, serial/parallel (S/P) converting section 21 converts data inputted in serial into parallel data, to obtain first symbol stream s1 and second symbol stream s2. First symbol stream s1 is outputted to first normalizing section 22 and STTD (Space Time Transmit Diversity) encoding section 24. In the same way, second symbol stream s2 is outputted to first normalizing section 22 and STTD encoding section 24.
First normalizing section 22 normalizes the two inputted symbol streams. For example, first normalizing section 22 normalizes symbols of the first symbol stream and second symbol stream by multiplying the first symbol stream and second symbol stream by a constant 1/√{square root over (2)}, such that the power of the symbols to be transmitted is 1, and outputs normalized symbol streams s1 and s2 to first spreading section 23.
First spreading section 23 spreads the inputted symbol streams. For example, first spreading section 23 multiplies the symbols by a specific spreading code to generate a first spreading sequence and second spreading sequence, and outputs the sequences to combining section 28.
On the other hand, STTD encoding section 24 performs the processing represented by following equation 1 on two inputted symbol streams s1 and s2, and outputs the results −s2* and s1* to second normalizing section 25.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                      (                                                                      s                  1                                                                                                      s                  2                                                              )                ⇒                  (                                                                      -                                      s                    2                    *                                                                                                                        s                  1                  *                                                              )                                    [        1        ]            
Second normalizing section 25 performs normalizing processing of multiplying inputted −s2* and s1* by a constant 1/√{square root over (2)}, for example, and outputs the result to second spreading section 26.
Second spreading section 26 performs spreading processing of multiplying the normalized symbol streams by a specific spreading code to generate a first spreading sequence and a second spreading sequence, and outputs the sequences to delay section 27.
Delay section 27 delays the first spreading sequence and the second spreading sequence by one chip, and outputs the delayed sequences to combining section 28.
Combining section 28 adds the first spreading sequence and second spreading sequence inputted from first spreading section 23 to the first spreading sequence and second spreading sequence inputted from delay section 27, and transmits the results from the respective two antennas.
The MIMO communication system employing the MPD method uses delay diversity of chip units, and so loses orthogonality of the spreading codes. Therefore, the interference signal of another communication terminal user (hereinafter simply “user”) remains in the despread signal, and so deteriorates the signal to interference noise ratio (SINR) of the despread signal. Further, also in flat fading, intersymbol interference is caused in the despread signal by the influence of delay. Therefore, with the MPD method, it is necessary to perform processing of canceling interference at the receiving side.
FIG. 3 shows a configuration of the receiving side of the conventional MIMO communication system employing the MPD method.
Despreading section 32 performs despreading processing of calculating the inner product of spreading code vector and the chip vector of the received signal on the received signal inputted through antenna 31.
Interference canceling section 33 cancels interference between users and intersymbol interference in the despread signal, and outputs the result to demodulating section 34.
Demodulating section 34 demodulates the inputted signal into first symbol streams and second symbol stream s2 using, for example, the minimum mean square error (MMSE) method.
Parallel/serial (P/S) converting section 35 converts the first symbol stream s1 and second symbol stream s2 inputted in parallel into serial streams, restores the original symbol streams and outputs the results.
As described above, in a MIMO communication system employing the MPD method, transmission diversity is realized by delay multiplexing.
Further, a MIMO communication system employing an STTD spreading method, which is an improved version of the MPD method, has also been proposed. By determining the space-time transmission order in chip units, the STTD spreading method makes it possible to ensure orthogonality of the transmission data in chip units and reduce intersymbol interference and interference between users due to delay of chip units at the receiving side. With the STTD spreading method, the spreading codes are orthogonal to each other, so that it is possible to substantially reduce complexity of detection. Like the MPD method, the MIMO communication system employing the STTD spreading method enables transmission diversity and space division multiplexing.    Non-Patent Document 1: “Multi-paths diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, R1-030565, Marne La Vallée, France, May 19th-23rd, 2003    Non-Patent Document 2: “Multi-paths diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, NY, R1-030760, New York, USA, August 25th-29th, 2003    Non-Patent Document 3: “Further results on Multi-Paths Diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, NY, R1-031102, Seoul, South Korea, Oct. 6th-11th, 2003    Non-Patent Document 4: “Rate Control for MPD”, 3GPP TSG RAN WG1, NY, R1-031316, Lisbon, Portugal, Nov. 17th-21st, 2003